Strontium tetraborate as optical glass material

ABSTRACT

Strontium tetraborate can be used as an optical material. Strontium tetraborate exhibits high refractive indices, high optical damage threshold, and high microhardness. The transmission window of strontium tetraborate covers a very broad range of wavelengths, from 130 nm to 3200 nm, making the material particularly useful at VUV wavelengths. An optical component made of strontium tetraborate can be incorporated in an optical system, such as a semiconductor inspection system, a metrology system, or a lithography system. These optical components may include mirrors, lenses, lens arrays, prisms, beam splitters, windows, lamp cells or Brewster-angle optics.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to and claims benefit of the earliestavailable effective filing date from the following applications: Thepresent application constitutes a continuation application of U.S.patent application Ser. No. 16/921,738, filed on Jul. 6, 2020, which isa regular (non-provisional) patent application of U.S. ProvisionalPatent Application No. 62/871,887, filed Jul. 9, 2019, whereby each ofthe above-listed applications are incorporated herein by reference inthe entirety.

TECHNICAL FIELD

The present disclosure relates generally to optical glass materials. Inparticular, the disclosure relates to an optical glass material,strontium tetraborate (SrB₄O₇), for linear optical components such asmirrors, lenses, prisms, beam splitters, windows and lamp cells suitablefor use in metrology and inspection systems in semiconductormanufacturing, including those used to inspect and/or measurephotomasks, reticles, and semiconductor wafers.

BACKGROUND

The integrated circuit industry requires inspection tools withincreasingly higher sensitivity to detect ever smaller defects andparticles whose sizes may be a few tens of nanometers (nm), or less.These inspection tools must operate at high speed in order to inspect alarge fraction, or even 100%, of the area of a photomask, reticle, orwafer, in a short period of time. For example, inspection time may beone hour or less for inspection during production or, at most, a fewhours for R&D or troubleshooting. In order to inspect so quickly,inspection tools use pixel or spot sizes larger than the dimensions ofthe defect or particle of interest, and detect just a small change insignal caused by a defect or particle. Detecting a small change insignal requires a high light level and a low noise level. High speedinspection is most commonly performed in production using inspectiontools operating with ultraviolet (UV) light. Inspection in R&D may beperformed with UV light or with electrons.

The integrated circuit (IC) industry also requires high precisionmetrology tools for accurately measuring the dimensions of smallfeatures down to a few nanometers or less on semiconductor wafers.Metrology processes are performed on wafers at various points in asemiconductor manufacturing process to measure a variety ofcharacteristics of the wafers such as a width of a patterned structureon the wafer, a thickness of a film formed on the wafer, and overlay ofpatterned structures on one layer of the wafer with respect to patternedstructures on another layer of the wafer. These measurements are used tofacilitate process controls and/or yield efficiencies in the manufactureof semiconductor dies. Metrology may be performed with UV light or withelectrons.

The semiconductor industry, which is aimed at producing integratedcircuits with higher integration, lower power consumption and lowercosts, is one of the main drivers of UV optics. The development ofpowerful UV light sources such as the excimer lasers and frequencymultiplied solid state lasers has led to the growth of research anddevelopment efforts in the field of UV photon applications.

Optical glasses are used in many applications such as cameras,telescopes, microscopes, binoculars, virtual reality systems,semiconductor systems, among others. Optical glasses are ubiquitous insemiconductor inspection and metrology. They are found in mostinspection and metrology systems in optical parts such as mirrors,lenses, prisms, beam splitters, windows and lamp cells.

Optical glasses in the deep ultraviolet (DUV), from approximately 200 nmto 280 nm, and vacuum ultraviolet (VUV), from approximately 100 nm to200 nm, spectral ranges are challenging. DUV and VUV lasers may havehigh power levels from several milli-watts (mW) to ten or more watts (W)and high photon energy (e.g., 6.5 eV at 193 nm and 4.66 eV at 266 nm).Pulsed lasers may have short pulse lengths (e.g., ns or less) and highrepetition rates (e.g., tens of kHz or greater). Optical glasses, inaddition to being transparent in the DUV/VUV wavelength ranges, need towithstand these extreme conditions with high optical damage threshold,high hardness and good stability.

There are a few glass materials known in the art suitable for DUV andVUV wavelengths. Among them, the most widely and commonly used is fusedsilica, which is known by trade names such as Suprasil, Spectrosil,Lithosil, etc. Fused silica is widely used due to cheap production sinceit is made from quartz and exhibits good thermal dimensional stabilityand durability. However, fused silica can only operate down to 190 nm inwavelength and most grades have a UV cutoff around 200 nm or longer.Optical parts made of fused silica are often smaller than 100 mm due tothe difficulty of finding UV grade blanks of that size or larger. Fusedsilica also has an absorption dip around 240 nm, which makes it a poorchoice for light sources operating in this region. In addition, theinternal structure of fused silica glass can be disrupted by longexposure to strong UV light.

Calcium fluoride (CaF₂) is another common UV glass material. CaF₂ istransparent from 130 nm to nearly 10 μm and has a low refractive index(n˜1.46) in the UV. Most fluorides are hygroscopic, that is, they absorbwater from the atmosphere. As a result, UV performance will decreaseover time when exposed to the atmosphere. Water will absorb the UVlight, and the absorption will cause a volume change leading to stressesand potential changes in shape. Furthermore, intense UV radiation canaccelerate reaction of water and oxygen with calcium fluoride. CaF₂material is soft and brittle, and it chips easily during polishing. Itis difficult to achieve high curvature and good surface roughnesstogether. CaF₂ also requires extensive cleanings between lapping becauseparticles from the lapping process can get trapped and rebonded to thesurface creating increased scattering sites and decreased overallperformance.

Magnesium fluoride (MgF₂) has a similar transmission window as CaF₂.MgF₂ is also the most common coating material for UV optics. UnlikeCaF₂, MgF₂ performance is not affected by water. However, MgF₂ has a lowoptical damage threshold (˜0.1 GW/cm²). Also, since MgF₂ is ionic it maynot withstand high voltages. In addition, MgF₂ is birefringent and maynot be appropriate for certain applications. When MgF₂ is exposed toultraviolet radiation, absorption may cause transmission to drop and itgets worse over time.

The damage thresholds of all the above materials are reduced byimpurities or defects in the material. These impurities and defects cancreate color centers that absorb DUV and/or VUV radiation. Color centerscan grow under exposure to VUV or DUV radiation causing transmission ofUV wavelengths to decrease over time.

While significant interest in producing stable glass materials under VUVand DUV illuminations has existed for several decades, there are only afew choices for optical glass materials in the DUV and VUV wavelengthranges. For the present application of high-speed inspection andmetrology, optical glasses need to have high optical damage thresholds,significant hardness and good stability. It is further desired that suchglasses have low permeability to diffusion of water and oxygen in orderto reduce oxidation.

Therefore, an optical glass material that overcomes some, or all, of thelimitations discussed above is desired.

SUMMARY

A linear optical component comprising strontium tetraborate (SrB₄O₇) isdisclosed, in accordance with one or more embodiments of the presentdisclosure.

An optical system is disclosed, in accordance with one or moreembodiments of the present disclosure. In one illustrative embodiment,the optical system includes one or more linear optical components,wherein at least a portion of the one or more linear optical componentsare formed from strontium tetraborate.

An optical system is disclosed, in accordance with one or moreadditional and/or alternative embodiments of the present disclosure. Inone illustrative embodiment, the optical system includes a stage forsupporting a sample. In another illustrative embodiment, the opticalsystem includes an illumination source. In another illustrativeembodiment, the optical system includes one or more linear opticalcomponents configured to direct illumination from the illuminationsource to the sample, wherein at least a portion of the one or morelinear optical components are formed from strontium tetraborate.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the invention as claimed. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1 illustrates a block diagram view of a characterization system, inaccordance with one or more embodiments of the present disclosure;

FIG. 2 illustrates an ultraviolet lamp incorporating SrB₄O₇ as anoptical glass material for one or more optical components, in accordancewith one or more embodiments of the present disclosure; and

FIG. 3 illustrates a typical transmission curve of SrB₄O₇.

DETAILED DESCRIPTION

The present disclosure has been particularly shown and described withrespect to certain embodiments and specific features thereof. Theembodiments set forth herein are taken to be illustrative rather thanlimiting. It should be readily apparent to those of ordinary skill inthe art that various changes and modifications in form and detail may bemade without departing from the spirit and scope of the disclosure.Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

Embodiments of the present disclosure are directed to the incorporationof strontium tetraborate (SrB₄O₇) as an optical glass material withinone or more linear optical components of semiconductor inspection and/ormetrology systems. For example, embodiments of the present disclosureincorporate SrB₄O₇ as an optical glass material in linear opticalcomponents found in inspection and metrology systems such as, but notlimited to, mirrors, lenses, lens arrays, prisms, beam splitters,windows and lamp cells.

It is noted that SrB₄O₇ exhibits unique optical and mechanicalproperties. The transparency range of SrB₄O₇ is 130-3200 nm inwavelength. See Y. S. Oseledchik, A. L. Prosvirnin, A. I. Pisarevskiy,V. V. Starshenko, V. V. Osadchuk, S. P. Belokrys, N. V. Svitanko, A. S.Korol, S. A. Krikunov, and A. F. Selevich, “New nonlinear opticalcrystals: strontium and lead tetraborates,” Opt. Mater. 4, 669 (1995),which is incorporated by reference herein in the entirety. This broadtransmission window makes SrB₄O₇ a good candidate for optical glassmaterial especially for the DUV and VUV wavelength ranges. If SrB₄O₇ isgrown in optimal conditions, the transmittance can reach more than 80%for wavelengths longer than 200 nm and more than 50% for 130 nm to 200nm. The refractive indices of SrB₄O₇ are high compared with other glassmaterials such as CaF₂. For example, the refractive indices at 266 nmare 1.7883 in the x direction, 1.7909 in the y direction and 1.7936 inthe z direction. Note that the differences among these refractiveindices are relatively small; thus, birefringence effects may be smallin components fabricated from crystalline SrB₄O₇. SrB₄O₇ is nothygroscopic, and since borate is an oxide, it is resistant to furtheroxidation if oxygen or water is present during exposure to DUV or VUVradiation. The optical damage threshold is very high (14.7 GW/cm²)compared with other glass materials. The surface laser-induced damagethreshold of SrB₄O₇ is about 16 J/cm², which is much higher than that offused silica and calcium fluoride. The microhardness of SrB₄O₇ is alsohigh (1750 kg/mm² in the x direction, 1460 kg/mm² in the y direction and1350 kg/mm² in the z direction). The high optical damage threshold andmicrohardness allow SrB₄O₇ glasses to withstand extreme conditions whenexposed to DUV and VUV radiation. DUV and VUV lasers may have high powerlevels from several milli-watts (mW) to several watts (W) or more, andhigh photon energy (for example 6.5 eV at 193 nm and 4.66 eV at 266 nm).Pulsed lasers may have short pulse lengths (ns or less) and highrepetition rates (tens of kHz or greater).

In accordance with embodiments of the present disclosure, one or morelinear optical components made of SrB₄O₇ disclosed herein may beincorporated into inspection and metrology systems. Semiconductorinspection tools must operate at high speed in order to inspect a largefraction, or even 100%, of the area of a photomask, reticle, or wafer,in a short period of time. For example, inspection time may be one houror less for inspection during production or, at most, a few hours forR&D or troubleshooting. In order to inspect so quickly, inspection toolsuse pixel or spot sizes larger than the dimensions of the defect orparticle of interest, and detect just a small change in signal caused bya defect or particle. High speed inspection is most commonly performedin production using inspection tools operating with ultraviolet (UV)light. High precision metrology tools are required for accuratelymeasuring the dimensions of small features down to a few nanometers orless on semiconductor wafers. Metrology processes are performed onwafers at various points in a semiconductor manufacturing process tomeasure a variety of characteristics of the wafers such as a width of apatterned structure on the wafer, a thickness of a film formed on thewafer, and an overlay offset of patterned structures on one layer of thewafer with respect to patterned structures on another layer of thewafer. These measurements are used to facilitate process controls and/oryield efficiencies in the manufacture of semiconductor dies. High-speedinspection and metrology require high light levels and a stable signal.Optical components are the building blocks of the inspection andmetrology systems. Optical glass materials that do not degrade, ordegrade more slowly than existing glass materials, can result in a morestable signal making it easier to detect small changes in signal. Suchglass materials also can reduce the operating cost of an inspection ormetrology tool by reducing the frequency of replacement of opticalcomponents.

FIG. 1 illustrates a simplified schematic view of a characterizationsystem 100, in accordance with one or more embodiments of the presentdisclosure. In one embodiment, the characterization system 100 (or“tool”) includes a characterization sub-system 101 and a controller 114.The characterization system 100 may be configured as an inspectionsystem or a metrology system. For example, the characterization system100 may be an optical-based inspection system (or “tool”), a reviewsystem (or “tool”), or an image-based metrology system (or “tool”). Inthis regard, the characterization sub-system 101 may be, but is notlimited to, an inspection sub-system or a metrology sub-systemconfigured to inspect or measure a sample 108. The characterizationsub-system 101 of the characterization system 100 may be communicativelycoupled to the controller 114. The controller 114 may receivemeasurement data from a detector assembly 104 of the characterizationsub-system in order to characterize (e.g., inspect or measure) astructure on or in sample 108 and/or control one or more portions of thecharacterization system 100.

Sample 108 may include any sample known in the art such as, but notlimited to, a wafer, reticle, photomask, or the like. In one embodiment,the sample 108 may be disposed on a stage assembly 112 to facilitatemovement of the sample 108. The stage assembly 112 may include any stageassembly known in the art including, but not limited to, an X-Y stage,an R-θ stage, and the like. In another embodiment, the stage assembly112 is capable of adjusting the height of the sample 108 duringinspection to maintain focus on the sample 108. In yet anotherembodiment, characterization sub-system 101 may be moved up and downduring inspection to maintain focus on the sample 108.

In another embodiment, the characterization system 100 includes anillumination source 102 configured to generate an illumination beam 111.The illumination source 102 may include any illumination source known inthe art suitable for generating an illumination beam 111. For example,the illumination source 102 may emit near infrared (NIR) radiation,visible radiation, ultraviolet (UV) radiation, near UV (NUV), deep UV(DUV) radiation, vacuum UV (VUV) radiation, and the like. For instance,the illumination source 102 may include one or more lasers. In anotherinstance, the illumination source 102 may include a broadbandillumination source.

In another embodiment, the characterization system 100 includes anillumination arm 107 configured to direct illumination from theillumination source 102 to the sample 108. The illumination arm 107 mayinclude any number and type of optical components known in the art. Inone embodiment, the illumination arm 107 includes one or more opticalelements 103. In this regard, illumination arm 107 may be configured tofocus illumination from the illumination source 102 onto the surface ofthe sample 108. It is noted herein that the one or more optical elements103 may include any optical element know in the art including, but notlimited to, one or more lenses (e.g., an objective lens 105), one ormirrors, one or more polarizers, one or more prisms, one or more beamsplitters, and the like.

In another embodiment, a collection arm 109 is configured to collectillumination reflected, scattered, diffracted, and/or emitted from thesample 108. In another embodiment, the collection arm 109 may directand/or focus the illumination from the sample 108 to a sensor 106 of adetector assembly 104. It is noted herein that sensor 106 and thedetector assembly 104 may include any sensor and detector assembly 104known in the art. It is noted that detector assembly 118 may include anysensor and detector assembly known in the art. The sensor may include,but is not limited to, charge-coupled device (CCD detector), acomplementary metal oxide semiconductor (CMOS) detector, a time-delayintegration (TDI) detector, a photomultiplier tube (PMT), an avalanchephotodiode (APD), a line sensor, an electron-bombarded line sensor, orthe like.

In another embodiment, the detector assembly 104 is communicativelycoupled to one or more processors 116 of the controller 114. The one ormore processors 116 may be communicatively coupled to memory 118. Theone or more processors 116 are configured to execute a set of programinstructions stored in memory 118 for acquiring measurement data fromthe one or more sensors 106 of the detector assembly 104 and/orcontrolling one or more portions of the characterization system 100.

In one embodiment, the characterization system 100 illuminates a line onsample 108 and collects scattered and/or reflected illumination in oneor more dark-field and/or bright-field collection channels. In thisembodiment, detector assembly 104 may include a line sensor or anelectron-bombarded line sensor.

In one embodiment, illumination source 102 is a continuous source. Forexample, the illumination source 102 may include, but is not limited to,an arc lamp, a laser-pumped plasma light source, or a continuous wave(CW) laser. In another embodiment, illumination source 102 is a pulsedsource. For example, the illumination source 102 may include, but is notlimited to, a mode-locked laser, a Q-switched laser, or a plasma lightsource pumped by a mode-locked or Q-switched laser. Examples of suitablelight sources that may be included in illumination source 102 aredescribed in U.S. Pat. No. 7,705,331, entitled “Methods and systems forproviding illumination of a specimen for a process performed on thespecimen”, to Kirk et al., U.S. Pat. No. 9,723,703, entitled “System andmethod for transverse pumping of laser-sustained plasma”, to Bezel etal., and U.S. Pat. No. 9,865,447, entitled “High brightnesslaser-sustained plasma broadband source”, to Chuang et al, which areeach incorporated by reference herein.

In one embodiment, the one or more optical elements 103 includes anillumination tube lens 133. The illumination tube lens 133 may beconfigured to image an illumination pupil aperture 131 to a pupil withinthe objective lens 105. For example, the illumination tube lens 133 maybe configured such that the illumination pupil aperture 131 and thepupil within the objective lens 105 are conjugate to one another. In oneembodiment, the illumination pupil aperture 131 may be configurable byswitching different apertures into the location of illumination pupilaperture 131. In another embodiment, the illumination pupil aperture 131may be configurable by adjusting a diameter or shape of the opening ofthe illumination pupil aperture 131. In this regard, the sample 108 maybe illuminated by different ranges of angles depending on thecharacterization (e.g., measurement or inspection) being performed undercontrol of the controller 114.

In one embodiment, the one or more optical elements 103 include acollection tube lens 123. For example, the collection tube lens 123 maybe configured to image the pupil within the objective lens 105 to acollection pupil aperture 121. For instance, the collection tube lens123 may be configured such that the collection pupil aperture 121 andthe pupil within the objective lens 105 are conjugate to one another. Inone embodiment, the collection pupil aperture 121 may be configurable byswitching different apertures into the location of collection pupilaperture 121. In another embodiment, the collection pupil aperture 121may be configurable by adjusting a diameter or shape of the opening ofcollection pupil aperture 121. In regard, different ranges of angles ofillumination reflected or scattered from the sample 108 may be directedto detector assembly 104 under control of the controller 114.

In another embodiment, the illumination pupil aperture 131 and/or thecollection pupil aperture 121 may include a programmable aperture.Programmable apertures are generally discussed in U.S. Pat. No.9,255,887, entitled “2D programmable aperture mechanism,” to Brunner,issued on Feb. 9, 2016; and U.S. Pat. No. 9,645,287, entitled “Flexibleoptical aperture mechanisms,” to Brunner, issued on May 9, 2017, both ofwhich are herein incorporated by reference in the entirety. Methods ofselecting an aperture configuration for inspection is generallydescribed in U.S. Pat. No. 9,709,510, entitled “Determining aconfiguration for an optical element positioned in a collection apertureduring wafer inspection,” to Kolchin et al., issued on Jul. 18, 2017;and U.S. Pat. No. 9,726,617, entitled “Apparatus and methods for findinga best aperture and mode to enhance defect detection,” to Kolchin et al,issued on Aug. 8, 2017, both of which are herein incorporated byreference in the entirety.

In one embodiment, one or more linear optical components of the optics103 and/or the illumination source 102 are formed from SrB₄O₇. Thelinear optical components herein may include mirrors, lenses, lensarrays, prisms, beam splitters, windows and/or lamp cells. The overalllight throughput of system 100 may be improved by appropriately usingSrB₄O₇as the optical glass material for one or more linear opticalcomponents. The lifetime of key linear optical components may also beimproved by using SrB₄O₇. A linear optical component may be fabricatedfrom a single SrB₄O₇ crystal, or from SrB₄O₇ glass. It is noted thatglass versions of the SrB₄O₇ linear optical components may be weaker(lower damage threshold, lower microhardness) than a correspondingsingle crystal form SrB₄O₇optical component. Nevertheless, SrB₄O₇ glassis adequately robust for many applications. It is further noted thatSrB₄O₇ glass possesses the additional advantage of displaying nobirefringence. Thus, SrB₄O₇ glass may be implemented in settings whereno birefringence is required (e.g., high numerical aperture lenses). Incases where the birefringence effects are minor (e.g., low numericalaperture lenses) or desirable (e.g., a polarizing beam splitter), singlecrystal SrB₄O₇ may be implemented and has the advantage of higherresistance to damage than glass.

In one embodiment, illumination beam 111 is polarized, for example, asin light generated by a laser. In this embodiment at least one of theone or more linear optical components of the optics 103 and theillumination source 102 is configured to operate substantially atBrewster's angle relative to the polarization direction of illuminationbeam 111. For example, the one or more linear optical components mayinclude one of Brewster's angle wavefront processing optics, aBrewster's angle lens, a Brewster's angle dual wavelength waveplate, andharmonic separation optics with a Brewster's angle input surface. Sincethe refractive index of SrB₄O₇ is higher than most other opticalmaterials, there may be no suitable material to use as an antireflectioncoating on SrB₄O₇ especially at DUV and VUV wavelengths. An advantage ofoptics configured to operate at Brewster's angle is that thereflectivity will be low without using any antireflection coating.Furthermore, antireflection coatings tend to be susceptible to damagewhen operated under high UV intensity. Avoiding an antireflectioncoating allows optical components fabricated from SrB₄O₇ to fullybenefit from the high damage threshold of SrB₄O₇. It is noted that thereflectivity of a surface is low for angles close to Brewster's angle.Deviations of a few degrees (such as about 2° or less) of theorientation of the surface from Brewster's angle result in very lowreflectivity. Brewster's angle for SrB₄O₇ crystals is about 60.5±1° overa wide range of visible and UV wavelengths. Because of this, Brewster'sangle optics fabricated from SrB₄O₇ may be used with polarizedbroad-band light as well as with laser light. More details of opticsconfigured to operate at Brewster's angle can be found in U.S. Pat. Nos.8,711,470, 9,152,008 and 9,753,352, all entitled “High Damage ThresholdFrequency Conversion System”, and all to Armstrong, which areincorporated herein by reference in their entirety.

Characterization systems are generally are described in U.S. Pat. No.9,891,177, entitled “TDI Sensor in a Darkfield System”, toVazhaeparambil et al., issued on Feb. 13, 2018; U.S. Pat. No. 9,279,774,entitled “Wafer Inspection”, to Romanovsky et al., issued on Mar. 8,2018; U.S. Pat. No. 7,957,066, entitled “Split Field Inspection SystemUsing Small Catadioptric Objectives,” to Armstrong et al., issued onJun. 7, 2011; U.S. Pat. No. 7,817,260, entitled “Beam Delivery Systemfor Laser Dark-Field Illumination in a Catadioptric Optical System,” toChuang et al., issued on Oct. 19, 2010; U.S. Pat. No. 5,999,310,entitled “Ultra-Broadband UV Microscope Imaging System with Wide RangeZoom Capability,” to Shafer et al., issued on Dec. 7, 1999; U.S. Pat.No. 7,525,649, entitled “Surface Inspection System Using Laser LineIllumination with Two Dimensional Imaging,” to Leong et al., issued onApr. 28, 2009; U.S. Pat. No. 9,080,971, entitled “Metrology Systems andMethods,” to Kandel et al., issued on Jul. 14, 2015; U.S. Pat. No.7,474,461, entitled “Broad Band Objective Having Improved Lateral ColorPerformance,” to Chuang et al., issued on Jan. 6, 2009; U.S. Pat. No.9,470,639, entitled “Optical Metrology With Reduced Sensitivity ToGrating Anomalies,” to Zhuang et al., issued on Oct. 18, 2016; U.S. Pat.No. 9,228,943, entitled “Dynamically Adjustable Semiconductor MetrologySystem,” to Wang et al., issued on Jan. 5, 2016; U.S. Pat. No.5,608,526, entitled “Focused Beam Spectroscopic Ellipsometry Method andSystem,” to Piwonka-Corle et al., issued on Mar. 4, 1997; and U.S. Pat.No. 6,297,880, entitled “Apparatus for Analyzing Multi-Layer Thin FilmStacks on Semiconductors,” to Rosencwaig et al., issued on Oct. 2, 2001,all of which are incorporated herein by reference in the entirety.

It is noted that the scope of the present disclosure is not limited tothe characterization system 100. Rather, the system incorporating theSrB₄O₇ optics of the present disclosure may include any other opticalsystem known in the art including a lithographic system/tool.

It is noted herein that the one or more components of system 100 may becommunicatively coupled to the various other components of system 100 inany manner known in the art. For example, the one or more processors 116may be communicatively coupled to each other and other components via awireline (e.g., copper wire, fiber optic cable, and the like) orwireless connection (e.g., RF coupling, IR coupling, WiMax, Bluetooth,3G, 4G, 4G LTE, 5G, and the like).

The one or more processors 116 may include any one or more processingelements known in the art. In this sense, the one or more processors 116may include any microprocessor-type device configured to executesoftware algorithms and/or instructions. The one or more processors 116may consist of a desktop computer, mainframe computer system,workstation, image computer, parallel processor, or other computersystem (e.g., networked computer) configured to execute a programconfigured to operate the system 100, as described throughout thepresent disclosure. It should be recognized that the steps describedthroughout the present disclosure may be carried out by a singlecomputer system or, alternatively, multiple computer systems.Furthermore, it should be recognized that the steps described throughoutthe present disclosure may be carried out on any one or more of the oneor more processors 116. In general, the term “processor” may be broadlydefined to encompass any device having one or more processing elements,which execute program instructions from memory 118. Moreover, differentsubsystems of the system 100 (e.g., illumination source 102, detectorassembly 104, controller 114, and the like) may include processor orlogic elements suitable for carrying out at least a portion of the stepsdescribed throughout the present disclosure. Therefore, the abovedescription should not be interpreted as a limitation on the presentdisclosure but merely an illustration.

The memory 118 may include any storage medium known in the art suitablefor storing program instructions executable by the associated one ormore processors 116 and the data received from the metrology sub-systemand/or inspection sub-system. For example, the memory 118 may include anon-transitory memory medium. For instance, the memory 118 may include,but is not limited to, a read-only memory (ROM), a random-access memory(RAM), a magnetic or optical memory device (e.g., disk), a magnetictape, a solid-state drive and the like. It is further noted that memory118 may be housed in a common controller housing with the one or moreprocessors 116. In an alternative embodiment, the memory 118 may belocated remotely with respect to the physical location of the processors116, controller 114, and the like. In another embodiment, the memory 118maintains program instructions for causing the one or more processors116 to carry out the various steps described through the presentdisclosure.

FIG. 2 illustrates a simplified schematic view of an ultraviolet lampthat incorporates SrB₄O₇ as an optical glass material for one or moreoptical components, in accordance with one or more embodiments of thepresent disclosure. The ultraviolet lamp 200 may be a laser-driven lightsource. In this example, a laser 211 emits laser beam 212, which isdirected by mirror 213 and focused by lens 214 and lens 225 andgenerates a plasma 202 inside a lamp cell 201. The plasma 202 emitsbroadband ultraviolet light 205 over a broad range of wavelengthsincluding DUV wavelengths and/or VUV wavelengths. One or more windows203 may be placed in a wall of the lamp cell 201 to enable broadbandultraviolet light 205 to be transmitted out of lamp cell 201. In oneembodiment, the lamp cell 201 may be made of SrB₄O₇. In this embodiment,SrB₄O₇ may be used to form a transparent bulb, which contains the gasfor generating plasma 202. In another embodiment, the one or morewindows 203 may be formed from SrB₄O₇. In yet another embodiment, boththe lamp cell 201 and the one or more windows 203 may be made of SrB₄O₇.The overall light throughput of ultraviolet lamp 200 may be improved byappropriately using SrB₄O₇ as the optical glass material for one or moreoptical components. The lifetime of ultraviolet lamp 200 and key opticalcomponents may also be improved by using SrB₄O₇. Any of theaforementioned optical components may be fabricated with SrB₄O₇ crystalor glass and the scope of the present disclosure is not at all limitedto the SrB₄O₇-based windows or plasma cells. Rather, as discussedpreviously herein, any number of linear optical components of thepresent disclosure may be formed from SrB₄O₇ and may be implemented inany optical context, which may include, but is not limited to,semiconductor inspection or metrology.

SrB₄O₇ crystallizes in the orthorhombic system, P2 ₁nm, with unit celldimensions a=4.237 Å, b=4.431 Å, and c=10.706 Å (A. Perloff, and S.Block, “The crystal structure of the strontium and lead tetraborates,SrO.2B₂O₃ and PbO.2B₂O₃,” Acta Cryst. 20, 274-279 (1966)). All boronatoms are coordinated tetrahedrally and an oxygen atom is common tothree tedrahedra. Despite the three-dimensional network of tetrahedral,the borate network appears as a layer-like structure since there arerelatively fewer links in the c direction of the unit cell.

FIG. 3 illustrates a typical transmission curve 300 of SrB₄O₇ (Y. S.Oseledchik, A. L. Prosvirnin, A. I. Pisarevskiy, V. V. Starshenko, V. V.Osadchuk, S. P. Belokrys, N. V. Svitanko, A. S. Korol, S. A. Krikunov,and A. F. Selevich, “New nonlinear optical crystals: strontium and leadtetraborates,” Opt. Mater. 4, 669 (1995)). As shown in transmissioncurve 300, the transparency range of SrB₄O₇ is very broad, namely fromabout 130 nm to about 3200 nm, which covers VUV, DUV, visible, and nearinfrared (IR) wavelength ranges. The VUV and DUV ranges are ofparticular interest to semiconductor inspection and metrology. It isalso noted that the transmittance is high. For instance, thetransmittance exceeds 80% from about 250 nm to about 2500 nm. This hightransmittance makes SrB₄O₇a good candidate for optical glass materialsespecially for the UV wavelength range. If SrB₄O₇ is grown in optimalconditions, a better transmission curve can be obtained: thetransmittance can reach more than 80% for wavelengths longer than 200 nmand more than 50% for 130 nm to 200 nm. Dielectric and opticalproperties of strontium tetraborate glasses are described by M. V.Shankar and K.B.R. Varma in “Dielectric and Optical Properties ofStrontium Tetraborate Glass,” Journal of Materials Science Letters 15(1996) 858-860, which is incorporated herein by reference in theentirety.

Various modifications to the described embodiments will be apparent tothose with skill in the art, and the general principles defined hereinmay be applied to other embodiments. Although it is expected that theoptical glass material disclosed herein will be particularly useful insemiconductor inspection and metrology systems, it is also envisionedthat these glass materials may be useful in other applications where VUVand DUV radiation are present, such as in an optical lithography system,and where high intensity visible or IR radiation is present, such as inan IR light source or camera system.

The glass material and methods described herein are not intended to belimited to the particular embodiments shown and described but are to beaccorded the widest scope consistent with the principles and novelfeatures herein disclosed.

One skilled in the art will recognize that the herein describedcomponents, operations, devices, objects, and the discussionaccompanying them are used as examples for the sake of conceptualclarity and that various configuration modifications are contemplated.Consequently, as used herein, the specific exemplars set forth and theaccompanying discussion are intended to be representative of their moregeneral classes. In general, use of any specific exemplar is intended tobe representative of its class, and the non-inclusion of specificcomponents, operations, devices, and objects should not be taken aslimiting.

The previous description is presented to enable one of ordinary skill inthe art to make and use the invention as provided in the context of aparticular application and its requirements. As used herein, directionalterms such as “top,” “bottom,” “over,” “under,” “upper,” “upward,”“lower,” “down,” and “downward” are intended to provide relativepositions for purposes of description, and are not intended to designatean absolute frame of reference. Various modifications to the describedembodiments will be apparent to those with skill in the art, and thegeneral principles defined herein may be applied to other embodiments.Therefore, the present invention is not intended to be limited to theparticular embodiments shown and described, but is to be accorded thewidest scope consistent with the principles and novel features hereindisclosed.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations are not expressly set forth herein for sakeof clarity.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, other components. It isto be understood that such depicted architectures are merely exemplary,and that in fact many other architectures can be implemented whichachieve the same functionality. In a conceptual sense, any arrangementof components to achieve the same functionality is effectively“associated” such that the desired functionality is achieved. Hence, anytwo components herein combined to achieve a particular functionality canbe seen as “associated with” each other such that the desiredfunctionality is achieved, irrespective of architectures or intermedialcomponents. Likewise, any two components so associated can also beviewed as being “connected,” or “coupled,” to each other to achieve thedesired functionality, and any two components capable of being soassociated can also be viewed as being “couplable,” to each other toachieve the desired functionality. Specific examples of couplableinclude but are not limited to physically mateable and/or physicallyinteracting components and/or wirelessly interactable and/or wirelesslyinteracting components and/or logically interacting and/or logicallyinteractable components.

Furthermore, it is to be understood that the invention is defined by theappended claims. It will be understood by those within the art that, ingeneral, terms used herein, and especially in the appended claims (e.g.,bodies of the appended claims) are generally intended as “open” terms(e.g., the term “including” should be interpreted as “including but notlimited to,” the term “having” should be interpreted as “having atleast,” the term “includes” should be interpreted as “includes but isnot limited to,” and the like). It will be further understood by thosewithin the art that if a specific number of an introduced claimrecitation is intended, such an intent will be explicitly recited in theclaim, and in the absence of such recitation no such intent is present.For example, as an aid to understanding, the following appended claimsmay contain usage of the introductory phrases “at least one” and “one ormore” to introduce claim recitations. However, the use of such phrasesshould not be construed to imply that the introduction of a claimrecitation by the indefinite articles “a” or “an” limits any particularclaim containing such introduced claim recitation to inventionscontaining only one such recitation, even when the same claim includesthe introductory phrases “one or more” or “at least one” and indefinitearticles such as “a” or “an” (e.g., “a” and/or “an” should typically beinterpreted to mean “at least one” or “one or more”); the same holdstrue for the use of definite articles used to introduce claimrecitations. In addition, even if a specific number of an introducedclaim recitation is explicitly recited, those skilled in the art willrecognize that such recitation should typically be interpreted to meanat least the recited number (e.g., the bare recitation of “tworecitations,” without other modifiers, typically means at least tworecitations, or two or more recitations). Furthermore, in thoseinstances where a convention analogous to “at least one of A, B, and C,and the like” is used, in general such a construction is intended in thesense one having skill in the art would understand the convention (e.g.,“a system having at least one of A, B, and C” would include but not belimited to systems that have A alone, B alone, C alone, A and Btogether, A and C together, B and C together, and/or A, B, and Ctogether, and the like). In those instances where a convention analogousto “at least one of A, B, or C, and the like” is used, in general such aconstruction is intended in the sense one having skill in the art wouldunderstand the convention (e.g., “a system having at least one of A, B,or C” would include but not be limited to systems that have A alone, Balone, C alone, A and B together, A and C together, B and C together,and/or A, B, and C together, and the like). It will be furtherunderstood by those within the art that virtually any disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be understood toinclude the possibilities of “A” or “B” or “A and B.”

What is claimed is:
 1. A linear optical component comprising strontiumtetraborate.
 2. The linear optical component of claim 1, wherein thestrontium tetraborate comprises strontium tetraborate in a glass phase.3. The linear optical component of claim 1, wherein the linear opticalcomponent comprises at least one of a mirror, a lens, a lens array, aprism, a beam splitter, a window, or a lamp cell.
 4. The linear opticalcomponent of claim 2, wherein the linear optical component is configuredto transmit light having a wavelength within a range from 130 nm to 400nm.
 5. A broadband ultraviolet lamp comprising: one or more linearoptical components, wherein at least a portion of the one or more linearoptical components are formed from strontium tetraborate.
 6. Thebroadband ultraviolet lamp of claim 5, wherein the strontium tetraboratecomprises strontium tetraborate in a glass phase.
 7. The broadbandultraviolet lamp of claim 5, wherein the one or more linear opticalcomponents comprise at least one of a mirror, a lens, a lens array, aprism, a beam splitter, a window, or a lamp cell.
 8. The broadbandultraviolet lamp of claim 5, wherein the broadband ultraviolet lamp isconfigured to emit light, the light having a wavelength within a rangefrom 130 nm to 400 nm.
 9. An optical system comprising: one or morelinear optical components, wherein at least a portion of the one or morelinear optical components are formed from strontium tetraborate.
 10. Theoptical system of claim 9, wherein the strontium tetraborate comprisesstrontium tetraborate in a glass phase.
 11. The optical system of claim9, wherein the one or more linear optical components comprise at leastone of a mirror, a lens, a lens array, a prism, a beam splitter, awindow, or a lamp cell.
 12. The optical system of claim 9, wherein theone or more linear optical components are configured to transmit lighthaving a wavelength within a range from 130 nm to 400 nm.